Insight Manager indeed is one giant source of information. Load your Management CD in the server, locate "cpqagin.nlm" on it, and run that one. Accept the defaults, and it will install the needed agents for you. Load the same CD on a client, and install "Compaq Insight Manager". When you run it, green lights are good, yellow lights are to be looked at, and red lights are bad. And it can show you graphs on CPU usage, network traffic, disk I/O etc.
In Monitor.nlm you'll find quite some useful information as well. In the "Lan/Wan information" menu, you can check if there are errors piling up on your network, or in "Storage Information" you can see if your disks start using hot-fix blocks. However, these things are less obvious to find and interpret, and they don't necessary always indicate a hardware problem.
Jim